Saw a drop in mpg from 45mpg to 34mpg. Since I was way over 200,000 miles, I began to fear traction battery failure. Toyota looked over codes, did diagnostics, etc. and found nothing. They recommended and replaced auxiliary battery, which made no difference. I fought with them on aux. battery replacement price since their costs had gone up 50% in two years. Mine was barely out of warrantee. So what happened. I replaced the iridium plugs. Mileage jumped back up to 45mpg.
so is it the iridium plugs or the change of plugs makes the difference? Iridium plugs appear to be much more expensive and I wonder if it gets similar result with other types of plugs.
Great news... my 60,000 mile 2007 has seen a drop from a VERY steady lifetime city/freeway combo of 45mpg down to about 35mpg over the past 6 or 7 tankfulls. The Toyota dealer said it could be so many things - like weather (I live in So Cal) or my driving style and so on... but I keep detailed records and I know something is different. Even when I just leave the gas station and pulse/glide I used to get easy 60+mpg fo the first few miles... now not so much. Guess I gotta check the plug - but would that have an effect even if/when I pulse/glide out of the gas station almost all downhill to my next regular stop? Any other thoughts?
One thing to make sure of is that your engine is actually shutting off when coasting. One of the symptoms of a bad 12V battery, which will not take a charge properly, is that it keeps the inverter running excessively and that keeps the engine running when it otherwise would shut off.
Iridium plugs cost at most $8 each vs. $2 for regular plugs. With the increased mileage they will pay for themselves on the first tank of gas. My wife's '06 Corolla uses Iridium plugs. I replaced them when they caused a warning light on the dashboard. Saved a $60 diagnosis charge from the dealer stealer.
The maintenance schedule called for the plugs change at 120k or 150k miles. So, 200k is a bit of a stretch. Do you still have the old plugs? I'll be interested to see the condition. I changed my 2006 Prius plug at 113k miles and they looked like this (left Denso is new). The gas mileage may have improved a little (0.5 or 1 MPG?) but it was barely noticeable, suggesting the plugs still had life.
I had a look at the plugs on my 3rd gen, last weekend, and I would not wish that chore on anyone (kudos to NutzAboutBolts for his step-by-step video). Toyota in their wisdom has made it impossible to extract the plugs without removing wipers, bottom-of-windshield trim, wiper motor/linkage and metal cowl (c/w brace). The actuators have a rigid snorkel piece running down to the spark plugs in the well. With a few inches increase in clearance to cowl, or redesign of the actuator to somehow make it flexible, none of the above tear-down would be needed. Anyway, the plugs looked great, at a mere 36000 km's.
